Ultrasonic measuring process for the wall thickness curve of a weld seam of a pipe

ABSTRACT

An ultrasonic measuring process is disclosed for measuring the wall thickness variation in the region of the welding seam of a pipe. At least two testing heads are each acoustically coupled to the pipe by a water jet. The acoustic entry zones of the testing heads are moved together in relation to the pipe, parallel to the axis of the pipe. At least one of the testing heads is moved back and forth across the axis of the tube, crossing over the welding seam. The acoustic entry zone can be moved by modulation of a phased array. The second testing head either emits acoustic radiation into the pipe always outside the welding seam or also crosses the welding seam, but at different times than the first testing head. The signals of both testing heads are processed in a measurement value processing circuit such that simultaneously occurring measurement value deviations supply no output signal.

The invention pertains to an ultrasonic measuring process for the wallthickness curve of a weld seam of a pipe, especially a pipe with ashaved weld seam, wherein an ultrasonic oscillator is coupledacoustically to the pipe via a hydraulic buffer, from it ultrasonicimpulses enter in an acoustic entry zone into the pipe, the reflectedcomponents of these pulses from the ultrasonic oscillator are receivedagain and fed as an electrical signal to a measured value processingcircuit, the ultrasonic oscillator is moved relative to the pipeparallel to the pipe axis and the acoustic entry zone is moved back andforth transversely to the pipe axis in such a way that the weld seam andthe peripheral regions to the left and right of the weld seam are passedover; the invention also pertains to devices for execution of thismeasurement process.

In a welded pipe, the wall thickness curve in the region of the weldseam generally deviates from the curve of the wall thickness of theremaining cross section of the pipe. The goal in the production of awelded pipe is that the wall thickness curve differ as little aspossible, preferably not at all, from the wall thickness curve outsideof the weld seam so that the welded pipe differs as little as possiblefrom an ideal cylindrical pipe.

Without reworking, the weld seam is generally thicker than the wallthickness of the coil from which the pipe is manufactured. In thisprocess it is attempted to have the weld seam protruding both to theoutside and to the inside relative to ideal cylinder geometry, i.e. tobe convex. The weld seam can be taken down by reworking, especiallyshaving, to such an extent that the ideal cylindrical curve is presenton the outside and the inside.

During welding, defects may appear; despite the greater wall thicknessin the region of the weld seam, the outside contour may run concavely,the inner contour will then have a relatively strong convex curvature.Furthermore, small irregularities in the longitudinal direction of theweld seam, e.g., waves or ribs, may appear.

As a result of the secondary treatment which is summarized in thefollowing under the concept of shaving, material is removed on theoutside and inside with the purpose of configuring the cross-sectionalcurve in such a way that an ideal purely cylindrical pipe geometry ispresent. The shaving is generally performed with a chisel: an externalchisel is used for outside shaving and an internal chisel is used forinside shaving. Both are arranged at a certain distance behind thewelding device of a conventional type and need not be discussed furtherhere. If deviations appear during the shaving, e.g., if the attitude ofthe chisel changes, the chisel wears down or breaks, the pipe geometrywill deviate from the sought ideal curve. Typical in such case areshaving operations which are conducted a little above the actual idealpipe contour and result in sharp edged "steps" in the cross-sectionalcurve of the pipe. Another deviation involves shaving off too much,causing the pipe then to be flattened out in the cross-sectional curveat the weld seam.

A weld seam test of the type mentioned initially is known which operatesby means of a hydraulic-coupled ultrasonic oscillator. This ultrasonicmeasurement process, however, does not operate with the accuracyrequired for testing of reworked weld seams. Thus it is difficult toregister the geometries of shaved pipes in which the wall thickness inthe curve of the weld seam although just as large as that beside theweld seam, nevertheless deviates from the ideal curve of cylindricalgeometry, because the outside shaving, for example, has causedflattening and a certain convexity was preserved during the insideshaving. But in particular, according to the previously known ultrasonicmeasurement procedures, steps, especially small steps in thecross-sectional curve of the pipe cannot be demonstrated with certainty.The transportation of the pipe by the conveyer mechanism always takesplace with a certain irregularity. The pipe being conveyed usuallyexecutes small movements transversely to its longitudinal axis which maybe periodical or nonperiodical. Periodical movements occur, for example,as a result of worn guide rolls. According to the previously knownprocess, it is impossible to distinguish between guidance inaccuraciesand geometric deviations of the same magnitude, e.g., small steps in thecross-sectional curve of the welded pipe.

The requirements imposed on the geometry of welded pipes are specifiedby the receiver and by the intended application purpose. If welded pipesare to be used, e.g., for exhaust systems, deviations from the idealgeometry may lead to flow variations in the exhaust gases which impairthe efficiency of the exhaust system, especially in an exhaust systemwith a catalyst.

In the ultrasonic measuring process of the type mentioned initially theultrasonic oscillator is not guided directly on the pipe. When theultrasonic oscillator is guided directly on the pipe a constant bufferdistance must always be present so that deviations from the idealcylindrical geometry, e.g., flattening due to outside shaving cannot beregistered. The previously known measurement process, therefore,operates with a movement device which moves the ultrasonic oscillatorwithout direct guidance, therefore freely, relative to the pipe. Thefree oscillation, however, has the disadvantage that the above describedchanges in position (e.g., vibrations) of the pipe also change thehydraulic buffer path and can therefore simulate geometry errors. Thesound is emitted radially so that the pipe wall is impinged vertically.

Starting with the measuring process of the type mentioned initially, theinvention has the task of avoiding the disadvantages of this knownprocess and the device operating according to it and improving both theprocess and the apparatus in such a way that the relative movements ofthe pipe transversely to the ideal (theoretical) central axis of thepipe have no effect on the measured value so that even small steps inthe wall thickness can be registered in the cross-sectional curve of areworked welded pipe.

This problem is solved by starting with the ultrasonic measuring processof the type mentioned initially by arranging a second ultrasonicoscillator, which is also coupled acoustically to the pipe via ahydraulic buffer, in the vicinity of the first ultrasonic oscillator,exposing the pipe to ultrasonic pulses and sending the reflectedcomponents of said pulses as electrical signals to the measured valueprocessing circuit, said second ultrasonic oscillator executing at leastaxial motion relative to the pipe together with the first ultrasonicoscillator; the two ultrasonic oscillators are never located above theweld seam at the same time, and the signals of the two ultrasonicoscillators are processed in the measured value processing circuit insuch a way that measured value deviations occurring within a time spanwhich is so brief that practically no additional mechanical change inposition of the pipe can take place in it deliver no output signal.

According to the invention, therefore, a second ultrasonic oscillator isprovided which is also not guided directly on the pipe and is heldeither independently of the first ultrasonic oscillator by its ownmechanism or is rigidly connected to it, e.g., in the form of twoindividual test heads or in the form of an array, especially a phasedarray. In the first case, its own mechanism, e.g., is stationary, sothat the second ultrasonic oscillator always scans the pipe on theoutside of the weld seam and is therefore capable of registeringpositional changes of the pipe. Any change in position registered by itis subtracted from the signal of the first ultrasonic oscillator in themeasured value processing circuit so that positional changes in the pipebeing tested have no effect on the output signal.

In the case of an also moving second ultrasonic oscillator, the movementmechanism of the second ultrasonic oscillator is matched to the movementmechanism of the first ultrasonic oscillator in such a way that theultrasonic oscillators pass over the weld seam at different times. Thetwo ultrasonic oscillators are arranged, when viewed in the crosssectional plane of the pipe, in such a way that they emit sound into thepipe along the pipe circumference in an offset manner. In addition, theymay also be offset with respect to one another axially (in the directionof the pipe). The offset should be selected to be as small as possible.The two ultrasonic oscillators are supposed to beam sound into the pipeat positions as close together as possible in order for the movements ofa partial region of the pipe relative to another partial region not tolead to a falsification of the measurement result.

In the case of a separate but not back-and-forth-moving secondultrasonic converter, the latter is used only for compensation of therelative movements of the pipe. If the second ultrasonic oscillator alsomoves back and forth, it also emits a signal over the geometric curve ofthe weld seam which, to be sure, is offset in time relative to thesignal of the first ultrasonic oscillator. The two signals can thereforebe combined with one another by a suitable known procedure in such a waythat a single statement is obtained concerning the curve of the weldseam at the measured position.

The arrangement of several ultrasonic oscillators into a linear arrayhas been found to be especially favorable, in which case the individualultrasonic oscillators are arranged tangentially to a radial plane ofthe pipe so that their central beams intersect the longitudinal axis ofthe pipe at a single point. Back and forth movement of the ultrasonicoscillator is not necessary; rather the array, preferably a phasedarray, is modulated electronically in such a way that the acoustic entryzone on the outer skin of the pipe is moved back and forth, passing overthe weld seam. The back and forth movement and the scanning behavior areselected in such a way that--as in the previously described case ofseparate ultrasonic oscillators--reference values outside of the weldseam and values from the weld seam itself are present. In this case thebriefest possible measuring time is selected so that between theregistration of a reference value and a measurement in the region of theweld seam no significant movement of the pipe can have taken place. Asopposed to the previously described design with two separate ultrasonicoscillators, which are preferably operated simultaneously, in the caseof an array the emission of two pulses at the same time is impossible.To be sure, two pulses can be emitted in such brief succession andreceived again that no significant movement of the pipe can haveoccurred in the intervening time. Pipe movements take place within atime greater than 0.1 s. Measured values which are collected in ashorter time are therefore practically not influenced by a pipemovement.

Relative changes in the pipe, therefore positional changes, vibrations,etc., cannot influence the results of the measurements according to theinvention. External shaving effects or other deviations from theexterior ideal cylindrical geometry are registered by a change in thehydraulic buffer, therefore the entry echo in the output signal is timeshifted. The wall thickness can be obtained by the known method from thetime difference between the entry echo and the back wall echo. In thisway the wall thickness curve can be represented. The wall thicknesscurve includes, on the one hand, deviations of the outer contour fromthe ideal cylindrical geometry and, on the other hand, deviations of theinner contour from the ideal cylindrical geometry, with both deviationsbeing registered independently of one another.

The problem posed is mechanically solved by an apparatus according tothe invention.

In a preferred modification, both ultrasonic oscillators are offsetcircumferentially with respect to each other at least by the width ofthe weld seam (shaving width). In this way only one ultrasonicoscillator will scan the reworked weld seam; there are never twoultrasonic oscillators simultaneously coupled to the shaved region.

Advantageously, the ultrasonic oscillators are acoustically coupled tothe pipe via water jets. Ultrasonic oscillators of this type with a freewater jet for contactless coupling are described, e.g., in the book byJ. and H. Krautkramer "Werkstoffprufung mit Ultraschall", 4th edition,Springer Verlag, in U.S. Pat. Nos. 3,255,626, 3,485,088, 3,908,455 and4,403,510 and in EP Patent 119 096. In an advantageous version, a commonsomewhat wider water jet can be used for both ultrasonic oscillators orat least both ultrasonic oscillators are connected via the samehydraulic conveyer so that the functional security of the installationis increased. The coupling of each individual ultrasonic oscillator canbe monitored by known methods by measuring the entry echo.

In mechanical terms, it has been found to be advantageous in the designof a jointly moving second ultrasonic oscillator to arrange the twoultrasonic oscillators on the same holder which executes a motion on anarc of circle around the axis of the pipe segment by means of a commonmovement mechanism. Relative movements of the two ultrasonic oscillatorswith respect to each other are prevented. Parallelogram guides have beenfound to be very suitable for the movement mechanism, especially sincethey can be adapted relatively rapidly to different pipe dimensions.

Other advantages and features of the invention may be obtained from theremaining claims and from the description below of examples ofembodiment of the invention which are not limiting and which areillustrated in detail with reference to the drawing, in which:

FIG. 1 shows a perspective view of an apparatus for ultrasonicmeasurement of the wall thickness curve of a weld seam of pipe,

FIG. 2 is a top view of a pipe as in FIG. 1 explaining the curve of thetest tracks which are plotted in,

FIG. 3 is a view corresponding to FIG. 2 for another version of themechanism,

FIG. 4 is a representation corresponding to FIG. 2 but for a mechanismwith three ultrasonic oscillators arranged in a V pattern.

FIG. 5 is a diagram of the time curve t of the entry of the echo intothe outer wall of the pipe and the inner wall of the pipe plottedagainst the test distance for two ultrasonic oscillators,

FIG. 6 is the wall thickness curve d found from the data in FIG. 5(solid lines) and the hydraulic buffer, again plotted against the testpath s, and

FIG. 7 is a perspective view as in FIG. 1 of a pipe being tested with atest head constructed from several ultrasonic oscillators in the form ofan array.

In FIG. 1 a mechanism is shown schematically for the ultrasonicmeasurement of the wall thickness curve of a weld seam 20 of a pipe 22.The pipe 22 is produced by the conventional method from a coil or othersuitable sheet metal blank in an endless process. The blank is curvedinto an open pipe, the narrow opposing edges are joined together alongthe weld seam 20, thus forming the pipe 22.

Shortly behind the welding mechanism (not shown, since it is well known)is an also not shown, since well known, mechanism for reworking. Itusually has an outer and an inner chisel. The outer chisel shaves theoutside and the inner chisel shaves the inside. The shaving is supposedto be performed in each case in such a way that the smallest possibledeviation from ideal pipe geometry appears in the region of the weldseam when viewed in cross section.

According to the process of the invention, the wall thickness curves inthe region of the weld seam 20 can be registered for production controlin order to avoid rejects due to defective welding, but especiallybecause of defective shaving.

The welded pipe 22 is situated on a conventional mechanism for guidanceand axial conveying which is not shown here in detail, the conveyingtaking place in the direction of the directional arrow 26 whichcoincides with the longitudinal pipe axis 24. The weld seam 20 issituated at the highest point on the pipe 22, generally described as the12 o'clock position. The weld seam may deviate from this position as aresult of production factors.

Above the pipe 22 are two ultrasonic oscillators 28, 30 which aredesigned as separate testing heads and which are both aligned in such away that they beam sound on a radial line into the walls of the pipe 22.Expressed in other terms, in operation of the ultrasonic oscillators 28,30, in each case the central beam of the emitted ultrasonic pulse moveson a line which runs at right angles to the longitudinal pipe axis 24and intersects it.

The two ultrasonic oscillators 28, 30 do not touch the pipe 22mechanically, rather they are located a free distance above the pipewhich amounts, e.g., to 5 to 50 millimeters. The ultrasonic oscillators28, 30 in each case are acoustically coupled to the pipe 22 via ahydraulic buffer 32 in the form of a water jet. Ultrasonic oscillatorswith water jet coupling are already well known and are called "bubblers"in the technical jargon. Through a common flexible hydraulic feed line34 the two ultrasonic oscillators 28, 30 are supplied with water, thewater flows through the rigid mechanical connection 42 of the twoultrasonic oscillators 28, 30 and then flows down in the direction ofthe main beam of the ultrasonic oscillator. In addition, each ultrasonicoscillator is electrically connected through a flexible lead wire 36 toa control unit 38 in which for each ultrasonic oscillator 28, 30 atransmitter and a measured value processing circuit 40 are situated. Thehydraulic line 34 and the feed line 36 are supported and carried by aboom.

Both ultrasonic oscillators 28, 30 can be adjusted by conventional meanswith respect to their rigid mechanical connecting piece 42 in the radialplane of the pipe 22 and transversely to it in order to achieve thedescribed alignment of the central beams. On the connecting piece 42 inthe center an arm 44 of a parallelogram arrangement 46 is rigidlyseated. The latter additionally has two parallel running follower arms48 of the same length hinge-jointed at one end to the arm 44 and onedrive arm 50 also hinge-jointed to the follower arms 48 running parallelto the arm 44. The latter is longer than the arm 44 and terminates in ahorizontal plane which runs through the longitudinal pipe axis 24 and ata distance the length of the follower arms 48 from the longitudinal pipeaxis 24. There the shaft of a drive motor 52 is situated which isdesigned as a stepping motor. The length of the arm 44 is selected suchthat the ultrasonic oscillators 28, 30 at the said free distance frompipe 22 are moved back and forth on a concentric segment of a circulararc. This is done by moving the driving arm 50 back and forth in thedirection of the arrow 54, the reciprocating movement of the ultrasonicoscillators 28, 30 themselves is represented by the double arrow 56. Themovement is carried out in such a way that both ultrasonic oscillators28, 30 cross the weld seam 20 and both sides of said weld seam 20register undisturbed regions of the pipe wall not influenced by thewelding. The ultrasonic oscillators 28, 30 operate in the pulse echomode; therefore they emit brief sound pulses whose reflections (echoes)are redetected and transformed into an electrical signal to be sent tothe measured value processing circuit 40.

The main beams of the two ultrasonic oscillators 28, 30 are situated inthe same radial plane, although offset along the circumference of thepipe 22. The distance from the acoustic entry zones 41, 43 of the twoultrasonic oscillators 28, 30 from one another is as small as possiblebut greater than the width of the weld seam 20.

The practical sequence of execution of the ultrasonic measuring processfor the wall thickness curve of a weld seam of a pipe is described inthe following, this description serving also to explain the ultrasonicmeasuring process: the pipe 22 is conveyed forward (out of the plane ofthe picture) in the direction of the arrow 26 and thus in the directionof its longitudinal axis 24. This conveying does not always take placefree of vibration, rather periodically and nonperiodically disturbancesarise, the pipe 22 may possibly change its position, vibrate,experiences jolts due to worn drive rolls, etc.

Due to the swiveling back and forth of the drive arm 50 driven by motor52 the two ultrasonic oscillators 28, 30 are moved on an arc of a circlewhose midpoint lies in the longitudinal axis 24 of the pipe. Duringforward motion they successively pass over the weld seam 20 and alsoduring the reverse motion. At this time the acoustic entry zones 41, 43describe a curve such as is shown in FIG. 2. This curve is, in otherwords the site of the points where the central beams strike the pipeskin. The paths shown by solid lines of the ultrasonic oscillators 28,30, called the test track in the following, consist in reality of alarge number of individual test points lying side by side, since pulsesare constantly being fed at a high frequency into the pipe 22.

As a result of an emitted ultrasonic pulse, the ultrasonic oscillator 28or 30 in each case receives the so called entry echo, therefore thereflection of the emitted pulse on the skin surface of the pipe 22 aswell as the so called back wall echo, i.e., the reflection on the innerwall of the pipe 22. The duration of time when an entry echo appearsafter the emission of the pulse depends on the length of the hydraulicbuffer 32. The duration of time when the back wall echo appears depends,in addition to the length of the hydraulic buffer 32, also on thethickness of the wall of the pipe 22. From the two measurement data,therefore the geometric location of the acoustic entry point into thepipe and the geometric location of the inner wall are known so that froma series of ultrasonic measurements (shots) the geometric curve of thewall thickness can be stated.

The two ultrasonic oscillators 28, 30 operate at the same time and inthe same rhythm. Due to the combined movements, i.e., the advance of thepipe 22 in the direction of the arrow 26 and the reciprocating motion ofthe ultrasonic oscillators 28, 30 in the transverse direction, thelatter describe essentially sinusoidal test tracks on the pipe 22.Because of their offset position in space relative to one another, theultrasonic oscillators 28, 30 cross the weld seam 20 at different timesand at different places. This arrangement is selected in such a way thatthe two ultrasonic oscillators 28, 30 never scan the weld seam 20 at thesame time. The output signals of the two ultrasonic oscillators 28, 30are processed in the circuit 40 for measured value processing in such away that measured value changes occurring simultaneously aredisregarded. They are commonly caused by vibrations during themanipulation of the pipe or else vibrations of the two ultrasonicoscillators 28, 30 together.

On the other hand, measured value deviations detected by only oneultrasonic oscillator but not by the other are contained in the outputsignal; they describe geometric variations which should be registered.If an ultrasonic oscillator has registered such a change, then the otherultrasonic oscillator should register a corresponding change eitheralready with a certain lag time before or with the same lag time after.The lag time is the time which passes from when the test track of anultrasonic oscillator 28 intersects a pipe skin line running parallel tothe longitudinal axis 24 of the pipe, e.g., an edge of the weld seamuntil the next ultrasonic oscillator cuts the same line. In the measuredvalue processing circuit 40, the data coming in successively with thistime lag can be summarized by the geometric curve of the weld seam,e.g., by an electrical delay of the signal of the front runningultrasonic oscillator or by measures in the software sector. In this waythe information on the weld seam is made more reliable.

Measurement results such as are supplied by the measured valueprocessing circuit 40 are represented in FIGS. 5 and 6 for the case of apipe defectively shaved on the inside. FIG. 5 shows the curve of thesound running times t for the entry echo (on the top in each case) andfor the inner wall echo (on the bottom in each case) for the firstultrasonic oscillator 28, and below it that for the following secondultrasonic oscillator 30, in each case plotted against the path s. Theoutput signals in each case also contain disturbances 58 which--sincethey occur at the same time for both ultrasonic oscillators--do notappear in the output signal which is shown in FIG. 6. FIG. 6 is arepresentation of the geometric curve of the pipe cross section in astretched (not round) representation. The measured length of thehydraulic buffer in each case is represented by crosshatched bars; thelocal thickness of the pipe wall determined after this is shown by thesolid bars. These solid bars are the image of the cross section of thepipe wall studied. One clearly recognizes a sharp edge 60 on the innerwall; a shaving defect is present here. A threshold value circuitconnected in series determines whether the edge 60 is a jump lyingoutside or inside the assigned tolerances. Depending on this the pipestudied is selected.

In the example shown in FIG. 3, the two ultrasonic oscillators 28, 30are not moved, rather by means of a mechanism according to FIG. 1, onlythe ultrasonic oscillator 28 is moved back and forth, the ultrasonicoscillator 30 emits outside of the weld seam 20 radially into the pipe22 and is held rigidly at a free distance from the pipe skin. Its testtrack is therefore a straight line running parallel to the longitudinalpipe axis 24 on the pipe jacket, as shown in FIG. 3. The secondultrasonic oscillator 30 in this design serves only to remove thedisturbances from the values of the first ultrasonic oscillator.

The arrangement shown in FIG. 4 with a total of three ultrasonicoscillators 28, 29, 30 designed as separate testing heads and mounted ona common connecting piece 42 as well as the ultrasonic oscillator shownin FIG. 1 are advantageous. As FIG. 4 shows, the three ultrasonicoscillators, as opposed to the example shown in FIG. 1, are also offsetaxially, arranged in a V pattern. The first ultrasonic oscillator 28located at the tip of the V is at the front in the direction of motion.The ultrasonic oscillator 30 arranged to the left and above it andbehind it in the direction of motion describes a test track, indicatedby points, which coincides exactly in the region of crossing of the weldseam 20 in its back movement with the test track of the first ultrasonicoscillator 28. An axial offsetting such as is shown in FIG. 2 does notoccur here. In other words, both ultrasonic oscillators 28, 28 coverexactly the same (obliquely running) cross section through the pipewall. This is true for the backward motion, therefore in the drawing inFIG. 4, the downward motion. The ultrasonic oscillator 29 is not usedfor the forward motion, i.e., the upward motion, but rather theultrasonic oscillator 30 arranged to the left below and behind theultrasonic oscillator 28. It is arranged at a geometrically identicaldistance to that of the ultrasonic oscillator 29 with the consequencethat the lag times are identical in each case. This second ultrasonicoscillator 30 now describes a test track shown as a broken line whichcoincides partially with that of the first ultrasonic oscillator 28 butwith the already discussed time lag.

This arrangement has the advantage, in addition to the already describedregistration of the same cross-sectional curve, that the measured valuesof the individual ultrasonic oscillators can be summarized into a commonmeasured value in simplified form, as is the case in the example shownin FIGS. 1 and 2. In the examples shown in FIGS. 1 and 2, it isnecessary for a common registration of the forward motions to allow forthe altered time relationships and sequence relative to the backwardmotion. This is not necessary in the example shown in FIG. 4.

In the arrangement in FIG. 7, several ultrasonic oscillators 28 through30 (of which only 8 are shown) form a test head in the form of an arraywhich is designed here as a phased array. The individual ultrasonicoscillators of the array are arranged on an arc of a circle whosemidpoint coincides with the longitudinal axis 24 of the pipe. Theircentral beams intersect at a point on the longitudinal axis 24 of thepipe. The array is acoustically coupled to the outer wall of the pipe 22via a hydraulic buffer (not shown). The array extends over an angularrange which is larger than that of the angular range occupied by theweld seam. This assures that the array can emit sound on both sidesoutside of the weld seam in the (undisturbed) region of the pipe 22.

In practical operation, an operating sequence is achieved as has alreadybeen described with reference to the first example. The ultrasonicoscillators, however, do not move: the array remains stationary. Thepipe 22 is moved in its axial direction (arrow 26). The acoustic entryzone 41 is controlled electronically in such a way that it travels backand forth over the weld seam, the location of all acoustic entry zones41, 43 is the scanning line on the outer skin, shown as a broken line.Depending on the scanning rate used the scanning is conducted in such away that either (according to the first example) scanning is performedprogressively back and forth or after every n-th scan another acousticentry zone 43 is moved to, which lies safely outside the weld seam 22,to obtain a reference value.

It should not go without mention that according to the stated processnot only weld seams 22 running parallel to the longitudinal axis 24 ofthe pipe but also weld seams lying on a helical line can be tested.

We claim:
 1. A method for ultrasonic measurement of a wall thicknesscurve of a welded pipe with a weld seam, the method comprising:couplinga first ultrasonic oscillator to said pipe via a hydraulic buffer, thefirst ultrasonic oscillator being operable to produce pulses along aradial line toward an outer surface of the pipe; subjecting said pipe tosaid ultrasonic pulses in an acoustic entry zone in said pipe; receivingreflected components of said ultrasonic pulses by said first ultrasonicoscillator and coupling the reflected components as electrical signalsto a measured value processing circuit; moving said first ultrasonicoscillator relative to said pipe, parallel to a pipe axis, and movingthe acoustic entry zone on the outer surface of the pipe, back and forthin such a way as to pass repetitively from sensing a peripheral regionon one side of the weld seam, over said weld seam to a peripheral regionon an opposite side of said weld seam, by said first ultrasonicoscillator; arranging a second ultrasonic oscillator, coupledacoustically to said pipe via a hydraulic buffer, in a vicinity of saidfirst ultrasonic oscillator, the second ultrasonic oscillator beingoperable to produce pulses along a radial line toward the outer surfaceof the pipe; exposing said pipe to the ultrasonic pulses of the secondultrasonic oscillator such that reflected components of said ultrasonicpulses of the second ultrasonic oscillator are also sent as electricalsignals to the measured value processing circuit; and, moving saidsecond ultrasonic oscillator axially relative to said pipe together withsaid first ultrasonic oscillator, such that only one of said first andsecond ultrasonic oscillators can ever be above and sensing the weldseam at any given time and that the electrical signals of said first andsecond ultrasonic oscillators are processed in said measured valueprocessing circuit in such a way that variations present in theelectrical signals which occur both in the electrical signal of saidfirst ultrasonic oscillator and also in the electrical signal of saidsecond ultrasonic oscillator, for a given position transverse of thepipe axis, do not appear in the output signal.
 2. The method accordingto claim 1, wherein said first and second ultrasonic oscillators arearranged sufficiently close together that a distance between saidacoustic entry zones is not greater than twice a width of said weldseam.
 3. The method according to claim 1, wherein said first and secondultrasonic oscillators are rigidly connected relative to one another andare moved together.
 4. The method according to claim 1, wherein saidfirst and second ultrasonic oscillators are moved at a distance of 3 to30 millimeters from an outer surface of the pipe, back and forth withoutcontacting it.
 5. The method according to claim 1, wherein said firstand second ultrasonic oscillators are offset geometrically and are movedrelative to the pipe such that said first and second ultrasonicoscillators successively describe a same test track upon crossing saidweld seam.
 6. The method according to claim 1, wherein said first andsecond ultrasonic oscillators are moved back and forth over said weldseam, successive incoming signals of said first and second ultrasonicoscillators being combined into a common signal over a curve of the wallthickness.
 7. The method according to claim 1, wherein a plurality ofsaid first and second ultrasonic oscillators are combined to form a testhead in the form of an array.
 8. An apparatus for implementation of anultrasonic measuring process, comprising:a mechanism for guidance andaxial conveying of a welded pipe and the apparatus relative to oneanother; a first ultrasonic oscillator arranged on a first moving devicehaving means for moving the first ultrasonic oscillator back and forthtransversely relative to a weld seam of said welded pipe, said firstultrasonic oscillator forming a hydraulic buffer between itself and saidwelded pipe, and being connected to a measured value processing circuitoperable to produce an output, said first ultrasonic oscillator beingoperable to produce pulses along a radial line toward an outer surfaceof the pipe; at least one second ultrasonic oscillator arranged in closeproximity to said first ultrasonic oscillator, said second oscillatorforming a hydraulic buffer between itself and said welded pipe and beingconnected to the measured value processing circuit, said secondultrasonic oscillator being operable to produce pulses along a radialline toward the outer surface of the pipe; means for passing at leastone of the first and second oscillators outside said weld seam back andforth across the longitudinal axis of said welded pipe while crossingsaid at least one of the first and second oscillators from sensing aperipheral region on one side of said weld seam, over said weld seam,into a peripheral region on an opposite side of said weld seam and onlyone of said first and second oscillators being over and sensing the weldseam at any given time; and, wherein variations in the electricalsignals coupled to said measured value processing circuit for a givenposition on the welded pipe and which occur both in the electricalsignal of said first ultrasonic oscillator and in the electrical signalof said second ultrasonic oscillator do not appear in the output.
 9. Anapparatus for implementation of an ultrasonic measuring process for awelded pipe having a weld seam, comprising:a mechanism for guidance andaxial conveying of the welded pipe and the apparatus relative to oneanother; a first ultrasonic oscillator providing an acoustic entry zoneon said welded pipe that is moved back and forth across said weld seamof said welded pipe from sensing a peripheral region on one side of theweld seam, over the weld seam, to a peripheral region on an oppositeside of the weld seam, said first ultrasonic oscillator forming ahydraulic buffer between itself and said welded pipe and being connectedto a measured value processing circuit providing an output, said firstultrasonic oscillator being operable to produce pulses along a radialline toward an outer surface of the pipe; a number of additionalultrasonic oscillators connected to form an array with said firstultrasonic oscillator and also forming a hydraulic buffer between saidadditional ultrasonic oscillators and said welded pipe, outputs of theadditional ultrasonic oscillators providing electrical signals coupledto said measured value processing circuit, said array formed from saidultrasonic oscillators being held stationary and covering an angularrange larger than an angular range of said weld seam, said additionalultrasonic oscillator being operable to produce pulses along a radialline toward an outer surface of the pipe; and, wherein in said measuredvalue processing circuit, variations first ultrasonic oscillator and inthe electrical signal of said additional ultrasonic oscillators for agiven position on the welded pipe do not appear in the output of themeasured value processing circuit.
 10. The apparatus of claim 9, whereinsaid array formed from said additional ultrasonic oscillators is curvedas an arc of a circle having a midpoint of curvature that lies on alongitudinal axis of said welded pipe, and center beams of saidadditional ultrasonic oscillators intersect at a point on saidlongitudinal axis of said welded pipe.
 11. The method of claim 1,wherein said welded pipe comprises a shaved weld seam.
 12. The method ofclaim 4, wherein said first and second ultrasonic oscillators are movedat a distance of 5 to 12 millimeters from the outer edge of the pipe.13. The method of claim 7, wherein said array is a phased array, andfurther comprising modulating the phased array for moving the acousticentry zone thereof, transversely of the pipe axis.
 14. The apparatus ofclaim 8, further comprising a second moving device upon which the secondultrasonic oscillator is arranged, the second moving device having meansfor moving the second ultrasonic oscillator back and forth transverselyrelative to a weld seam of said welded pipe, wherein said second movingdevice is identical in size with said first moving device.
 15. Theapparatus of claim 9, wherein said array formed by said additionalultrasonic oscillators is a phased array and further comprising meansfor modulating the phased array such that the acoustic entry zone of thearray is moved transversely of a pipe axis.